1. FIELD OF THE INVENTION
The present invention relates generally to a matrix type display device and particularly to a selecting electrode drive circuit suitable for use in a matrix liquid crystal display such as a liquid crystal television.
A matrix type liquid crystal display is well known as consisting essentially of two electroded-transparencies separated by a sealed-in liquid crystal material such as a twisted nematic type liquid crystal or a guest-host type liquid crystal. The outer surface of each transparency has a transparent conductive coating in the form of parallel rows or columns. The electroded transparencies face each other with their row and column electrodes crosswise to each other, thereby forming a rectangular array of intersections of their row and column electrodes. Thus, a matrix results. In operation, a series of pulses at regular intervals are applied to the row electrodes, that is, selecting electrodes, one after another, thus sequentially bringing these selecting electrodes to a given potential for a very short period.
On the other hand, video signals representing a piece of visual information to be displayed are applied to the column electrodes, that is, data electrodes, thereby darkening the liquid crystal at selected intersections enough to form a visible shape.
With a view to extending the life of the liquid crystal used in a matrix type liquid crystal display device, the selection of the electrodes is performed by alternately using positive-going pulses and negative-going pulses described as follows. In interfaced scanning, alternate electrodes are driven by positive-going pulses in one frame and the remaining electrodes by negative-going pulses in the next frame. Otherwise, the selection is performed by applying alternating pulses at regular intervals to the selecting electrodes one after another.
There has been an ever increasing demand for improving the resolution of a matrix type liquid crystal display device. The resolution of the display will increase with an increase in the number of selecting (row) and data (column) electrodes. The increase in the number of selecting data electrodes, however, will lead to a decrease of the duration of each driving pulse enough to assure that driving pulses do not bring adjacent selecting electrodes to their active states at the same moment. In order to assure that selected intersections of the liquid are darkened enough to form a visible image, the decrease of pulse duration will have to be compensated for by increasing the amplitude or height of pulse. As a matter of fact, the amplitude of each driving pulse needs to be multiplied by the square root of the number of rows or columns.
A selecting electrode drive circuit usually uses C-MOS devices whose breakdown voltage ranges from about 20 to 22 volts. This figure of breakdown voltage of the semiconductor device accordingly limits the resolution of a matrix type liquid crystal display.
With a view to expanding the limit of resolution beyond the one permitted by the breakdown voltage of semiconductor devices used in a matrix type display device, the object of the present invention is to provide a selecting electrode drive circuit which is capable of providing driving pulses of twice the amplitude of the breakdown voltage of semiconductor devices, thus permitting a substantial increase in the number of selecting and data electrodes in the matrix type display device, and hence a corresponding increase of resolution in the matrix type display device.